Rotary internal combustion engine with pilot subchamber

ABSTRACT

A rotary engine including a rotor sealingly received within an internal cavity of an outer body to define a plurality of combustion chambers having a variable volume, a pilot subchamber located in a wall of the outer body, the pilot subchamber in fluid communication with the internal cavity via at least two spaced apart transfer holes defining a flow restriction between the pilot subchamber and the internal cavity, a pilot fuel injector in fluid communication with the pilot subchamber, an ignition element configured for igniting fuel in the pilot subchamber, and a main fuel injector extending through the stator body and communicating with the cavity at a location spaced apart from the pilot subchamber. A method of combusting fuel in a rotary engine is also discussed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of U.S. application Ser. No.14/706,457 filed May 7, 2015, which is a continuation of U.S.application Ser. No. 13/273,534 filed Oct. 14, 2011, which claimspriority from provisional U.S. application No. 61/512,593 filed Jul. 28,2011, the entire contents of all of which are incorporated by referenceherein.

TECHNICAL FIELD

This application relates generally to rotary internal combustionengines, more particularly to the combustion in such engines.

BACKGROUND OF THE ART

Rotary engines, such as for example Wankel engines, use the eccentricrotation of a piston to convert pressure into a rotating motion, insteadof using reciprocating pistons. In these engines, the rotor includes anumber of apex or seal portions which remain in contact with aperipheral wall of the rotor cavity of the engine throughout therotational motion of the rotor to create a plurality of rotatingchambers when the rotor rotates.

Wankel engines are typically used with gasoline or similar fuel, with asingle fuel injector or with two spaced apart fuel injectors. The fuelinjector(s) may be located in a recess adjacent the combustion chamberand defined integrally through the engine housing, to communicate withan ignition member such as for example a spark plug. However, knownarrangements are not optimized for use in a compound cycle engine systemand/or for use with so-called heavy fuels, such as kerosene, and thusroom for improvement exists.

SUMMARY

In one aspect, there is provided a rotary engine comprising a statorbody having an internal cavity defined by two axially spaced apart endwalls and a peripheral wall extending between the end walls, the cavityhaving an epitrochoid shape defining two lobes, a rotor body havingthree circumferentially spaced apex portions, the rotor body beingengaged to an eccentric portion of a shaft to rotate and perform orbitalrevolutions within the cavity with each of the apex portions remainingin sealing engagement with the peripheral wall and separating threerotating chambers of variable volume defined in the cavity around therotor body, an insert in the peripheral wall of the stator body, theinsert being made of a material having a greater heat resistance thanthat of the peripheral wall, the insert having a subchamber definedtherein and having an inner surface bordering the cavity, the subchambercommunicating with the cavity through at least one opening defined inthe inner surface and having a shape forming a reduced cross-sectionadjacent the opening, a pilot fuel injector having a tip received in thesubchamber, an ignition element having a tip received in the subchamber,and a main fuel injector extending through the stator body and having atip communicating with the cavity at a location spaced apart from theinsert.

In another aspect, there is provided a stator body for a Wankel enginecomprising two axially spaced apart end walls, a peripheral wallextending between the end walls and defining an internal cavitytherewith, the cavity having an epitrochoid shape defining two lobes, aninsert in the peripheral wall of the stator body, the insert being madeof a material having a greater heat resistance than that of theperipheral wall, the insert having a subchamber defined therein andhaving an inner surface bordering the cavity, the subchambercommunicating with the cavity through at least one opening defined inthe inner surface and having a shape forming a reduced cross-sectionadjacent the opening, at least one of the insert and the peripheral wallhaving a pilot fuel injector elongated hole defined therethroughcommunicating with the subchamber and sized to receive a pilot fuelinjector therein, at least one of the insert and the peripheral wallhaving an ignition element elongated hole defined therethroughcommunicating with the subchamber and sized to receive an ignitionelement therein, and the peripheral wall having a main fuel injectorelongated hole defined therethrough spaced apart from the insert andsized to receive a main fuel injector therein.

In yet another aspect, there is provided a method of injecting heavyfuel into a Wankel engine having rotating chambers each having a volumevarying between a minimum volume and a maximum volume, the methodcomprising injecting a minor portion of the heavy fuel into a subchamberdefined adjacent to and in sequential communication with each of therotating chambers and having a subchamber volume corresponding to from5% to 25% of a sum of the minimum volume and the subchamber volume,igniting the heavy fuel within the subchamber, partially restricting aflow of the ignited heavy fuel from the subchamber to the rotatingchambers, and injecting a remainder of the heavy fuel into each of therotating chambers sequentially, independently of and spaced apart fromthe subchamber.

In another aspect, there is provided a rotary engine comprising: a rotorbody mounted for eccentric revolutions within a stator body to providerotating chambers of variable volume in an internal cavity of the statorbody, the volume of each chamber varying between a minimum volume and amaximum volume; an insert in a peripheral wall of the stator body, theinsert being made of a material having a greater heat resistance thanthat of the peripheral wall, the insert having a subchamber definedtherein and having an inner surface, the subchamber communicating withthe cavity through at least one opening defined in the inner surface andhaving a shape forming a reduced cross-section adjacent the opening, thesubchamber having a volume corresponding to from 5% to 25% of a sum ofthe minimum volume and the volume of the subchamber; a pilot fuelinjector having a tip received in the subchamber, the tip of the pilotfuel injector extending through an injector opening defined through theinsert; an ignition element received within an ignition element holedefined through the insert, the ignition element having a tip receivedin the subchamber; and a main fuel injector extending through the statorbody and having a tip communicating with the cavity at a location spacedapart from the insert.

In another aspect, there is provided a method of injecting heavy fuelinto a Wankel engine having rotating chambers each having a volumevarying between a minimum volume and a maximum volume, the methodcomprising: injecting a minor portion of the heavy fuel into asubchamber defined adjacent to and in sequential communication with eachof the rotating chambers, the subchamber having a subchamber volumecorresponding to from 5% to 25% of a sum of the minimum volume and thesubchamber volume; igniting the heavy fuel within the subchamber;creating a hot wall around the subchamber by providing the subchamber inan insert received in a wall of a stator of the engine, the insert beingmade of a material more resistant to high temperature than that of thewall; partially restricting a flow of the ignited heavy fuel from thesubchamber to the rotating chambers; and injecting a remainder of theheavy fuel into each of the rotating chambers sequentially,independently of and spaced apart from the subchamber.

In another aspect, there is provided a rotary engine comprising: a rotorsealingly received within an internal cavity of an outer body to definea plurality of combustion chambers; a pilot subchamber located in a wallof the outer body, the pilot subchamber in fluid communication with theinternal cavity via at least two spaced apart transfer holes defining aflow restriction between the pilot subchamber and the internal cavity,the pilot subchamber in use in fluid communication with the combustionchambers as the rotor rotates; a pilot fuel injector in fluidcommunication with the pilot subchamber; an ignition element configuredfor igniting fuel in the pilot subchamber; and a main fuel injectorcommunicating with the internal cavity independently from the pilotsubchamber.

In another aspect, there is provided a compound engine assemblyincluding the rotary engine as defined above, a compressor in fluidcommunication with the intake port of the rotary engine, and a turbinein fluid communication with the exhaust port of the rotary engine, theturbine having a turbine shaft compounded with an engine shaft drivinglyengaged to the rotor.

In a further aspect, there is provided a method of combusting fuel in arotary engine, the method comprising: injecting a minor portion of thefuel into a pilot subchamber; injecting a major portion of the fuel intoa combustion chamber of the rotary engine independently of the pilotsubchamber; igniting the fuel within the pilot subchamber; and ignitingthe fuel in the combustion chamber by directing the ignited fuel fromthe pilot subchamber into the combustion chamber in at least twodiscrete flows.

DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures in which:

FIG. 1 is a partial, schematic cross-sectional view of a rotary internalcombustion engine in accordance with a particular embodiment;

FIG. 2 is a schematic cross-sectional view of an insert of the engine ofFIG. 1;

FIG. 3 is a schematic cross-sectional view of an insert in accordancewith another embodiment;

FIG. 4 is a schematic cross-sectional view of an insert in accordancewith a further embodiment;

FIG. 5 is a schematic cross-sectional view of an engine assembly inaccordance with a particular embodiment, in which the rotary internalcombustion engine of FIG. 1 can be used;

FIG. 6 is a schematic view of part of an inner surface of a wall of therotary engine of FIG. 1 in accordance with a particular embodiment; and

FIG. 7 is a schematic view of an end wall of a rotary engine of therotary engine of FIG. 1 in accordance with another particularembodiment.

DETAILED DESCRIPTION

Referring to FIG. 1, a rotary internal combustion engine 10 known as aWankel engine is schematically and partially shown. In a particularembodiment, the rotary engine 10 is used in a compound cycle enginesystem or compound cycle engine such as described in Lents et al.'s U.S.Pat. No. 7,753,036 issued Jul. 13, 2010, or such as described in Julienet al.'s U.S. Pat. No. 7,775,044 issued Aug. 17, 2010, or such asdescribed in Thomassin et al.'s U.S. patent publication No. 2015/0275749published Oct. 1, 2015, or such as described in Bolduc et al.'s U.S.patent publication No. 2015/0275756 published Oct. 1, 2015, the entirecontents of all of which are incorporated by reference herein. Thecompound cycle engine system may be used as a prime mover engine, suchas on an aircraft or other vehicle, or in any other suitableapplication. In any event, in such a system, air is compressed by acompressor before entering the Wankel engine, and the engine drives oneor more turbine(s) of the compound engine. In another embodiment, therotary engine 10 is used without a turbocharger, with air at atmosphericpressure. It is understood that the compound cycle engine system mayhave any other suitable configuration, including, but not limited to, asingle shaft engine assembly.

FIG. 5 illustrates a compound engine assembly 10 in accordance with aparticular embodiment, which may be configured for example as aturboprop engine, a turboshaft engine, a turbofan engine, or anauxiliary power unit (APU). The engine assembly 100 generally includes acompressor 120, an intermittent internal combustion engine 112configured for example as a liquid cooled heavy fueled multi-rotorrotary intermittent combustion engine, and a turbine section 118including one or more turbines.

The outlet of the compressor 120 is in fluid communication with theinlet of the engine 112; although not shown, such communication may beperformed through an intercooler so as to reduce the temperature of thecompressed air prior to the compressed air entering the engine 112. Inthe embodiment shown, the compressor 120 includes variable inlet guidevanes 123 through which the air flows before reaching the rotor(s) ofthe compressor 120. The compressor 120 may be a single-stage device or amultiple-stage device and may include one or more rotors having radial,axial or mixed flow blades.

A source of fuel 114 is in fluid communication with fuel injectors 42,78 (further described below) of the engine 112. In a particularembodiment, the source of fuel 114 is a source of heavy fuel e.g.diesel, kerosene, jet fuel, equivalent biofuel; other suitable types offuel may alternately be used, including, but not limited to, “lightfuel” such as gasoline and naphtha. In the engine 112 the compressed airis mixed with the fuel and combusted to provide power and a residualquantity of exhaust gas. The engine 112 drives an engine shaft 116, andprovides an exhaust flow in the form of exhaust pulses of high pressurehot gas exiting at high peak velocity. The outlet of the engine 112 isin fluid communication with the inlet of the turbine section 118, andaccordingly the exhaust flow from the engine 112 is supplied to theturbine(s) of the turbine section 118.

The turbine section 118 includes at least one turbine rotor engaged on aturbine shaft 119. In a particular embodiment, the turbine section 118includes a first stage turbine 122 receiving the exhaust from the engine112, and a second stage turbine 124 receiving the exhaust from the firststage turbine 122; each turbine 122, 124 may be a single-stage device ora multiple-stage device and may include one or more rotors havingradial, axial or mixed flow blades. In a particular embodiment, theturbines 122, 124 have different reaction ratios from one another. In aparticular embodiment, the first stage turbine 122 is configured to takebenefit of the kinetic energy of the pulsating flow exiting the engine112 while stabilizing the flow and the second stage turbine 124 isconfigured to extract energy from the remaining pressure in the flow.Accordingly, in a particular embodiment the reaction ratio of the firststage turbine 122 is lower than the reaction ratio of the second stageturbine 124. Other configurations are also possible.

Power from the engine 112 and turbines 122, 124 is compounded to drive arotatable load 108, for example via a gearbox 126 defining a drivingengagement between the engine shaft 116, the turbine shaft 119 and therotatable load 108. The rotatable load 108 may be any suitable type ofload including, but not limited to, one or more generator(s),propeller(s), helicopter rotor mast(s), fan(s), compressor(s), or anyother appropriate type of load or combination thereof. It is understoodthat the power from the engine shaft 116 and turbine shaft 119 may becompounded using any other suitable type of engagement, including, butnot limited to, by having each shaft engaged to a respective electricalmotor/generator with power being transferable between the electricalmotor/generators (electrical compounding).

In the embodiment shown, the compressor 120 is driven by the turbinesection 118, by having the rotor(s) of the compressor 120 directlyengaged to the turbine shaft 119. Alternately, the rotor(s) of thecompressor 120 may be connected to a separate shaft driven by theturbine shaft 119 and/or the engine shaft 116, for example via thegearbox 126 or via a separate gearbox.

It is understood that the engine assembly 100 may have otherconfigurations than that of a compound engine assembly. For example, theturbine section 118 may be omitted, or may rotate independently of theinternal combustion engine 112. The compressor 120 may be omitted. Forexample, the internal combustion engine 112 may have its inlet andoutlet in direct communication with ambient air, i.e. be used withoutbeing fluidly connected to a compressor and a turbine.

In the embodiment shown, the engine 112 is a rotary intermittentinternal combustion engine including two or more rotor assemblies 111drivingly engaged to the engine shaft 116. In another embodiment, theengine 112 includes a single rotor assembly 111. In a particularembodiment, the rotor assembly(ies) 111 are configured as Wankelengines.

Referring back to FIG. 1, an example of a Wankel engine 10 which maydefine a rotor assembly 111 of the engine 112 is shown. It is understoodthat the configuration of the rotor assembly 111, e.g. placement ofports, number and placement of seals, number of apex portions,combustion chambers, etc., may vary from that of the embodiment shown.

The engine 10 comprises an outer body 12 having axially-spaced end walls14 with a peripheral wall 18 extending therebetween to form a rotorcavity 20. The inner surface 19 of the peripheral wall 18 of the cavity20 has a profile defining two lobes, which is preferably an epitrochoid.

An inner body or rotor 24 is received within the cavity 20, with thegeometrical axis of the rotor 24 being offset from and parallel to theaxis of the outer body 12. The rotor 24 has axially spaced end faces 26adjacent to the outer body end walls 14, and a peripheral face 28extending therebetween. The peripheral face 28 defines threecircumferentially-spaced apex portions 30 (only one of which is shown),and a generally triangular profile with outwardly arched sides. The apexportions 30 are in sealing engagement with the inner surface ofperipheral wall 18 to form three rotating combustion chambers 32 (onlytwo of which are partially shown) between the inner rotor 24 and outerbody 12. A recess 38 is defined in the peripheral face 28 of the rotor24 between each pair of adjacent apex portions 30, to form part of thecorresponding chamber 32.

The combustion chambers 32 are sealed. Each rotor apex portion 30 has anapex seal 52 extending from one end face 26 to the other and protrudingradially from the peripheral face 28. Each apex seal 52 is biasedradially outwardly against the peripheral wall 18 through a respectivespring. An end seal 54 engages each end of each apex seal 52, and isbiased against the respective end wall 14 through a suitable spring.Each end face 26 of the rotor 24 has at least one arc-shaped face seal60 running from each apex portion 30 to each adjacent apex portion 30,adjacent to but inwardly of the rotor periphery throughout its length. Aspring urges each face seal 60 axially outwardly so that the face seal60 projects axially away from the adjacent rotor end face 26 intosealing engagement with the adjacent end wall 14 of the cavity. Eachface seal 60 is in sealing engagement with the end seal 54 adjacent eachend thereof.

Although not shown in the Figures, the rotor 24 is journaled on aneccentric portion of a shaft and includes a phasing gear co-axial withthe rotor axis, which is meshed with a fixed stator phasing gear securedto the outer body co-axially with the shaft. The shaft rotates the rotor24 and the meshed gears guide the rotor 24 to perform orbitalrevolutions within the rotor cavity. The shaft rotates three times foreach complete rotation of the rotor 24 as it moves around the rotorcavity 20. Oil seals are provided around the phasing gear to preventleakage flow of lubricating oil radially outwardly thereof between therespective rotor end face 26 and outer body end wall 14.

At least one inlet port (not shown) is defined through one of the endwalls 14 or the peripheral wall 18 for admitting air (atmospheric orcompressed) into one of the combustion chambers 32, and at least oneexhaust port (not shown) is defined through one of the end walls 14 orthe peripheral wall 18 for discharge of the exhaust gases from thecombustion chambers 32. The inlet and exhaust ports are positionedrelative to each other and relative to the ignition member and fuelinjectors (further described below) such that during each rotation ofthe rotor 24, each chamber 32 moves around the cavity 20 with a variablevolume to undergo the four phases of intake, compression, expansion andexhaust, these phases being similar to the strokes in areciprocating-type internal combustion engine having a four-strokecycle.

In a particular embodiment, these ports are arranged such that therotary engine 10 operates under the principle of the Miller or Atkinsoncycle, with its volumetric compression ratio lower than its volumetricexpansion ratio. In another embodiment, the ports are arranged such thatthe volumetric compression and expansion ratios are equal or similar toone another.

An insert 34 is received in a corresponding hole 36 defined through theperipheral wall 18 of the outer body 12, for pilot fuel injection andignition. The peripheral wall 18 also has a main injector elongated hole40 defined therethrough, in communication with the rotor cavity 20 andspaced apart from the insert 34. A main fuel injector 42 is received andretained within this corresponding hole 40, with the tip 44 of the maininjector 42 communicating with the cavity 20 at a point spaced apartfrom the insert 34. The main injector 42 is located rearwardly of theinsert 34 with respect to the direction R of the rotor rotation andrevolution, and is angled to direct fuel forwardly into each of therotating chambers 32 sequentially with a tip hole pattern designed foran adequate spray.

Referring particularly to FIG. 2, the insert includes an elongated body46 extending across a thickness of the peripheral wall 18, with anenlarged flange 48 at its outer end which is biased away from a shoulder50 defined in the peripheral wall 18, and against a gasket (not shown)made of an appropriate type of heat resistant material such as a silicabased material. A washer 56, such as for example a steel or titaniumwasher, and spring 58, such as for example a wave spring or a Bellevillespring, are provided between the flange 48 and the shoulder 50 of theperipheral wall 18. The spring 58 biases the body 46 against a cover 62having a cross-section greater than that of the hole 36 and extendingover an outer surface 64 of the peripheral wall 18. The cover 62 isconnected to the peripheral wall 18, for example through brazing.Alternate types of connections can also be used, including but notlimited to a connection through fasteners such as bolts, to helpfacilitate replacement of the insert if necessary.

The insert body 46 has an inner surface 66 which is continuous with theinner surface 19 of the peripheral wall 20 to define the cavity 20. Theinsert hole 36 in the wall 18 defines a flange 68 extending in theinsert hole 36 adjacent the inner surface 19, and the inner end of theinsert body 46 is complementarily shaped to engage this flange 68, witha gasket 70 being received therebetween.

The insert body 46 is made of a material having a greater heatresistance than that of the peripheral wall 18, which in a particularembodiment is made of aluminium. In this particular embodiment, theinsert body 46 is made of an appropriate type of ceramic.

The insert body 46 has a pilot subchamber 72 defined therein incommunication with the rotor cavity 20. In the embodiment shown, thesubchamber 72 has a circular cross-section; alternate shapes are alsopossible. The subchamber 72 communicates with the cavity through atleast one opening 74 defined in the inner surface 66. The subchamber 72has a shape forming a reduced cross-section adjacent the opening 74,such that the opening 74 defines a restriction to the flow between thesubchamber 72 and the cavity 20. The opening 74 may have various shapesand/or be defined by a pattern of multiple transfer holes.

Referring to FIG. 6, an example of communication between the subchamber72 and the internal cavity including multiple spaced apart transferholes 75 is shown. In the embodiment shown, four separate transfer holes75 are provided; it is understood that more or less transfer holes 75may alternately be provided. In the embodiment shown, the transfer holes75 are located in a diamond pattern; it is understood that any othersuitable pattern may alternately be used.

Referring back to FIG. 2, the peripheral wall 18 has a pilot injectorelongated hole 76 defined therethrough in proximity of the insert 34,extending at a non-zero angle with respect to a surface of an outer wallof the insert 34, and in communication with the subchamber 72. A pilotfuel injector 78 is received and retained within the corresponding hole76. The pilot fuel injector 78 is in fluid communication with thesubchamber 72, for example by having the tip 80 of the pilot injector 78received in the subchamber 72. As can be seen in FIG. 2, the insert body46 has an injector opening defined therethrough providing thecommunication between the pilot injector elongated hole 76 and thesubchamber 72, and the tip 80 of the pilot injector 78 is received inthe subchamber 72 through this injector opening, with a major part ofthe pilot injector 78 being received in the pilot injector elongatedhole 76 outside of the insert 34.

The insert body 46 and cover 62 have an ignition element elongated hole82 defined therein extending along the direction of the transverse axisT of the outer body 12, also in communication with the subchamber 72. Anignition element 84 is received and retained within the correspondinghole 82 and is configured for fuel ignition in the subchamber 72, forexample by having the tip 86 of the ignition element 84 received in thesubchamber 72. In the embodiment shown, the ignition element 84 is aglow plug in heat exchange relationship with the subchamber 72.Alternate types of ignition elements 84 which may be used include, butare not limited to, plasma ignition, laser ignition, spark plug,microwave, etc.

The pilot injector 78 and main injector 42 inject fuel, and in aparticular embodiment heavy fuel (e.g. diesel, kerosene, jet fuel,equivalent biofuel) into the chambers 32; the pilot injector 78 injectsa minor portion of the fuel while the main injector 42 injects a majorportion of the fuel. In a particular embodiment, at least 0.5% and up to20% of the fuel is injected through the pilot injector 78, and theremainder is injected through the main injector 42. In anotherparticular embodiment, at most 10% of the fuel is injected through thepilot injector 78. In another particular embodiment, at most 5% of thefuel is injected through the pilot injector 78. The main injector 42injects the fuel such that each rotating chamber 32 when in thecombustion phase contains a lean mixture of air and fuel.

Referring to FIG. 3, an insert 134 according to another embodiment isshown, engaged to the same outer body 12. The insert 134 extends acrossa thickness of the peripheral wall 18, and includes an inner bodyportion 146 and an outer body portion 162 which are attached together,for example through a high temperature braze joint 188. The outer bodyportion 162 has an enlarged flange 148 at its outer end which abuts theouter surface 64 of the peripheral wall 18 and is connected thereto, forexample through bolts with appropriate sealing such as a gasket or crushseal (not shown). Alternate types of connections can also be used,including but not limited to a brazed connection.

The inner body portion 146 has an inner surface 166 which is continuouswith the inner surface 19 of the peripheral wall 18 to define the cavity20. The inner end of the inner body portion 146 is complementarilyshaped to engage the flange 68 extending in the insert hole 36 adjacentthe inner surface 19, with a gasket 70 being received therebetween.

In this particular embodiment, the body portions 146, 162 are made of anappropriate type of super alloy such as a Nickel based super alloy.

The pilot subchamber 72 is defined in the insert 134 at the junctionbetween the body portions 146, 162, with the inner body portion 146defining the opening 74 for communication between the subchamber 72 andthe cavity 20. The outer body portion 162 has the ignition elementelongated hole 82 defined therein along the direction of the transverseaxis T and in communication with the subchamber 72. The ignition element84 is received and retained within the corresponding hole 82, forexample through threaded engagement. As in the previous embodiment, thetip 86 of the ignition element 84 is received in the subchamber 72.

Referring to FIG. 4, an insert 234 according to another embodiment isshown. The insert 234 is received in a corresponding hole 236 definedthrough the peripheral wall 18. The insert 234 includes an inner bodyportion 246 and an outer body portion 262 which are attached together,for example through a high temperature braze joint, with the subchamber72 being defined at the junction of the two portions 246, 262. The innerbody portion 246 defines the opening 74 for communication between thesubchamber 72 and the cavity 20.

The outer body portion 262 has the ignition element elongated hole 82defined therethrough in communication with the subchamber 72. The outerbody portion 262 includes an inner enlarged section 245 connected to theinner body portion 246 and defining the subchamber 72. The enlargedsection 245 extends substantially across the width of the hole 236around the subchamber 72, then tapers to a reduced width section 247extending therefrom. The reduced width section 247 has at its outer endan enlarged flange 248 which abuts a shoulder 250 defined in the outersurface 64 of the peripheral wall 18 around the hole 236. An outersection 249, which in the embodiment shown has a width intermediate thatof the sections 245 and 247, extends outwardly from the flange 248. Theflange is connected to the shoulder, for example through bolts (notshown) with appropriate sealing such as a crush seal or a gasket (notshown) made of high temperature material, for example a silica basedmaterial or grafoil, between the flange 248 and shoulder 250. Alternatetypes of connections can also be used.

The inner body portion 246 has an inner surface 266 which is continuouswith the inner surface 19 of the peripheral wall 18 to define the cavity20. The inner body portion 246 includes a groove defined therearoundnear the inner surface 266, in which an appropriate seal 251, forexample a silica based gasket tape, is received in contact with thewalls of the insert hole 236. In this embodiment, the walls of theinsert holes 236 are straight adjacent the inner surface 19, i.e. thereis no flange adjacent the inner surface 19.

The volume of the subchamber 72 in the insert 34, 134, 234 is selectedto obtain a stoichiometric mixture around ignition within an acceptabledelay, with some of the exhaust product from the previous combustioncycle remaining in the subchamber 72. In a particular embodiment, thevolume of the subchamber 72 is at least 0.5% and up to 3.5% of thedisplacement volume, with the displacement volume being defined as thedifference between the maximum and minimum volumes of one chamber 32. Inanother particular embodiment, the volume of the subchamber 72corresponds to from about 0.625% to about 1.25% of the displacementvolume.

The volume of the subchamber 72 may also be defined as a portion of thecombustion volume, which is the sum of the minimum chamber volume Vmin(including the recess 38) and the volume of the subchamber V₂ itself. Ina particular embodiment the subchamber 72 has a volume corresponding tofrom 5% to 25% of the combustion volume, i.e. V₂=5% to 25% of(V₂+V_(min)). In another particular embodiment, the subchamber 72 has avolume corresponding to from 10% to 12% of the combustion volume, i.e.V₂=10% to 12% of (V₂+V_(min)).

The subchamber 72 may help create a stable and powerful ignition zone toignite the overall lean main combustion chamber 32 to create thestratified charge combustion. The subchamber 72 may improve combustionstability, particularly but not exclusively for a rotary engine whichoperates with heavy fuel below the self-ignition of fuel. The insert 34,134, 234 made of a heat resistant material may advantageously create ahot wall around the subchamber which may further help with ignitionstability.

It is understood that the subchamber 72 may alternately be defined in oradjacent one of the end walls 14, as illustrated by FIG. 7. In thisembodiment, the fluid communication between the subchamber 72 and theinternal cavity is defined via three separate transfer holes extendingthrough the end wall 14. It is understood that more or less transferholes 75 may alternately be provided. In the embodiment shown, thetransfer holes 75 are located in a linear pattern; it is understood thatany other suitable pattern may alternately be used.

It is also understood that the subchamber 72 may be defined directly inthe peripheral wall 18 or end wall 14, i.e. without being defined in aninsert made of a different material, or may be defined in any othersuitable type and configuration of insert. For example, the embodimentsof FIGS. 6-7 include, but are not intended to be limited to, the insertconfigurations shown in FIGS. 1-4.

In a particular embodiment and in use, the fuel is combusted inaccordance with the following. A minor portion of the fuel is injectedinto the pilot subchamber 72, and a major portion of the fuel isinjected into a combustion chamber 32 independently of the pilotsubchamber 72. The fuel within the pilot subchamber 72 is ignited, forexample with the ignition element 84. The fuel in the combustion chamber32 by directing the ignited fuel from the pilot subchamber 72 into thecombustion chamber 32. In a particular embodiment, this is performed inat least two discrete flows, via the separate transfer holes 75 (FIGS.6-7) each defining one of the flows. Other configurations are alsopossible.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that changes may be made to the embodimentsdescribed without departing from the scope of the invention(s)disclosed. For example, the mechanical arrangement of the Wankel enginedescribed above is merely one example of many possible configurationswhich are suitable for use with the present invention(s). Any suitableinjector configuration and arrangement may be used. Hence, modificationswhich fall within the scope of the present invention will be apparent tothose skilled in the art, in light of a review of this disclosure, andsuch modifications are intended to fall within the appended claims.

The invention claimed is:
 1. A rotary engine comprising: a rotorsealingly received within an internal cavity of an outer body to definea plurality of combustion chambers; a pilot subchamber located in a wallof the outer body, the pilot subchamber in fluid communication with theinternal cavity via at least two spaced apart transfer holes defining aflow restriction between the pilot subchamber and the internal cavity,the pilot subchamber in use in fluid communication with the combustionchambers as the rotor rotates; a pilot fuel injector in fluidcommunication with the pilot subchamber; an ignition element configuredfor igniting fuel in the pilot subchamber; a main fuel injectorcommunicating with the internal cavity independently from the pilotsubchamber; and wherein the pilot subchamber has a volume correspondingto from 5% to 25% of a sum of a minimum volume of one of the combustionchambers and the volume of the pilot subchamber.
 2. The engine asdefined in claim 1, wherein an insert is received in a peripheral wallof the stator body, the insert being made of a material having a greaterheat resistance than that of the peripheral wall, the insert having thepilot subchamber defined therein, the at least two transfer holesextending through the insert.
 3. The engine as defined in claim 2,wherein the insert is made of ceramic or super alloy.
 4. The engine asdefined in claim 2, wherein the ignition element is received within anignition element hole defined through the insert.
 5. The engine asdefined in claim 2, wherein the pilot injector extends through theperipheral wall at a non-zero angle with respect to a surface of anouter wall of the insert with only a portion thereof extending withinthe insert.
 6. The engine as defined in claim 2, wherein the pilotinjector extends through the peripheral wall at a non-zero angle withrespect to a longitudinal direction of the insert.
 7. The engine asdefined in claim 1, wherein the volume of the pilot subchambercorresponds to from 10% to 12% of the sum of the minimum volume and thevolume of the pilot subchamber.
 8. The engine as defined in claim 1,wherein a difference between a maximum volume and a minimum volume ofone of the combustion chambers defines a displacement volume, thesubchamber having a volume of at least about 0.5% of the displacementvolume and at most about 3.5% of the displacement volume.
 9. The engineas defined in claim 1, wherein a difference between a maximum volume anda minimum volume of one of the combustion chambers defines adisplacement volume, the pilot subchamber having a volume of at least0.625% of the displacement volume.
 10. The engine as defined in claim 1,wherein a difference between a maximum volume and a minimum volume ofone of the combustion chambers defines a displacement volume, the pilotsubchamber having a volume of about 1.25% of the displacement volume.11. The engine as defined in claim 1, further comprising a heavy fuelsource in communication with the main and pilot fuel injectors.
 12. Theengine as defined in claim 1, wherein the ignition element is a glowplug.
 13. The rotary engine as defined in claim 1, wherein the rotaryengine is a Wankel engine, the outer body including a peripheral wallcooperating with two spaced apart end walls to define the internalcavity, the peripheral wall defining two lobes, the rotor having threecircumferentially-spaced apex portions in sealing engagement with theperipheral wall and separating the combustion chambers.
 14. A compoundengine assembly including the rotary engine as defined in claim 1, acompressor in fluid communication with the intake port of the rotaryengine, and a turbine in fluid communication with the exhaust port ofthe rotary engine, the turbine having a turbine shaft compounded with anengine shaft drivingly engaged to the rotor.
 15. The engine as definedin claim 1, wherein each of the at least two spaced apart transfer holeshaving a central axis located outside a periphery of each of the othersof the at least two spaced apart transfer holes.
 16. A method ofcombusting fuel in a rotary engine, the method comprising: injecting aminor portion of the fuel into a pilot subchamber; injecting a majorportion of the fuel into a combustion chamber of the rotary engineindependently of the pilot subchamber; igniting the fuel within thepilot subchamber; igniting the fuel in the combustion chamber bydirecting the ignited fuel from the pilot subchamber into the combustionchamber in at least two discrete flows; and wherein the pilot subchamberhas a volume corresponding to from 5% to 25% of a sum of a minimumvolume of the combustion chamber and the volume of the pilot subchambervolume.
 17. The method as defined in claim 16, wherein the volume of thepilot subchamber corresponds to from 10% to 12% of the sum of theminimum volume and the volume of the pilot subchamber.
 18. The method asdefined in claim 16, further comprising creating a hot wall around thepilot subchamber by providing the pilot subchamber in an insert receivedin a wall of a stator of the engine, the insert being made of a materialmore resistant to high temperature than that of the wall.
 19. The methodas defined in claim 18, wherein the insert is received in a peripheralwall of the engine.
 20. The method as defined in claim 18, wherein theinsert is made of ceramic or super alloy.
 21. The method as defined inclaim 18, wherein injecting the minor portion of the fuel into the pilotsubchamber includes injecting from 0.5 to 20% of the fuel.
 22. Themethod as defined in claim 16, wherein injecting the minor portion ofthe fuel into the pilot subchamber includes injecting from 5 to 10% ofthe fuel.
 23. The method as defined in claim 16, wherein injecting theminor portion of the fuel is done in an angled direction with respect toa central transverse axis of a stator of the engine.
 24. The method asdefined in claim 16, wherein the fuel is heavy fuel.
 25. The method asdefined in claim 16, wherein the rotary engine is a Wankel engine andthe combustion chamber is one of three combustion chambers, the enginehaving a housing including a peripheral wall cooperating with two spacedapart end walls to define an internal cavity, the peripheral walldefining two lobes, and having a rotor including threecircumferentially-spaced apex portions in sealing engagement with theperipheral wall and separating the three combustion chambers.
 26. Themethod as defined in claim 16, wherein directing the ignited fuel fromthe pilot subchamber into the combustion chamber in at least twodiscrete flows is via at least two spaced apart transfer holes, each ofthe at least two spaced apart transfer holes having a central axislocated outside a periphery of each of the others of the at least twospaced apart transfer holes.